A key step in the assembly of electronic systems is the attachment and interconnection of electronic devices to bonding pads of a circuit pattern defined on a substrate which can be, for example, ceramic, silicon or FR-4 epoxy. In accordance with modern soldering techniques, solder elements are formed on the bonding pads of the substrate. A bonding pad array of the electronic device to be bonded to the substrate is contacted to the solder elements. A flux, such as rosin, is required for bonding. The assembly is then heated to a temperature sufficient to reflow or melt the solder to effect a bond between the arrays of bonding pads which, after the solder is hardened, constitutes both an electrical interconnection and a structural bond. The rosin flux is required to assure a dependable solder bond and, after the solder has been hardened, rosin flux residue is normally cleaned from the electronic assembly.
The most effective cleaning solvents for removing rosin flux residues are chlorofluorocarbon (CFC) based solvents. Because of their harmful effect on the environment, however, considerable work has been done to replace CFC solvents with more benign alternatives. Alternative solvents and alternative cleaning methods must of course be sufficiently effective in removing rosin residue that any remaining residue does not interfere with device performance. It has therefore become customary to specify a maximum quantity of rosin flux residue that is permitted to be left after a cleaning operation and to monitor certain electronic assemblies to determine that the allowed maximum has not been exceeded. If the monitoring shows that the maximum has been exceeded, then the cleaning operation is revised to make the cleaning more thorough.
As described, for example, in the paper, "Cleaning and Cleanliness Test Program Phase I Test Results," Guidelines Report, IPC, Lincolnwood, Ill., various methods that have been used for monitoring residue include Solid Insertion Probe Fourier Transform Mass Spectrometry (FTMS), Scanning Electron Microscope Electron Dispersive Spectrometry (SEM EDS), and Fourier Transform Infrared Spectrometry (FTIR). The method that is described as being generally preferred, however, is High Performance Liquid Chromatography (HPLC), which is deemed to be more practical for production line use.
Our work with HPLC has indicated that the method, as specified, significantly underestimates the quantity of rosin flux residue left on an electronic assembly after cleaning. Accordingly, there is still a need for better methods to assure that rosin flux residue has been properly cleaned, and specifically for quantitative methods for monitoring residue left after cleaning.